The present invention relates to hard contact lenses having an excellent oxygen permeability.
Contact lenses put presently on the market are classified into two large groups, i.e. soft contact lenses made of a water-absorptive polymer such as poly(2-hydroxyethyl methacrylate) or a soft hydrophobic polymer such as silicone rubber, and hard contact lenses made of a hard polymer such as poly(methyl methacrylate). The hard contact lenses are generally inferior in wearing sensation to the soft contact lenses, but they have excellent visual power correcting effect and durability and also have the advantages peculiar to the hard contact lenses such as easiness in handling, and accordingly they are widely used at present.
Hard contact lenses have the fatal defect that it is difficult to supply oxygen required in metabolism of corneal tissue from the atmosphere to cornea through the lens materials, in other words, the oxygen permeability is bad. Consequently, wearing of the hard contact lenses for a long period of time causes metabolic trouble of the corneal tissue. In recent years, however, the above problem has been dissolved to some extent by the appearance of an oxygen permeable hard contact lens made of a copolymer of methyl methacrylate and a special methacrylate compound having siloxane bond (Si-O bond) in its molecule, and the hard contact lenses have rised in clinical estimation.
However, the proposed copolymer of the methacrylate compound having siloxane bond is inferior in hardness and rigidity to poly(methyl methacrylate) used as a material of usual hard contact lenses, and also is fragile. Lack of hardness and rigidity not only makes it easy to take scratches on the lens surface and makes it difficult to prepare contact lenses of a constant quality according to a predetermined lens contour, but also invites the fatal defect that a stable absolute visual power is not obtained due to difficulty in maintaining a constant lens contour upon wearing. On the other hand, the fragility results in lowering of the durability and rise in frequency of breaking as well as lowering of the mechanical processability. Accordingly, in order to obtain oxygen permeable hard contact lenses having an improved fragility as well as hardness and rigidity desired for hard contact lenses, the proportion of methyl methacrylate must be increased while decreasing the proportion of the siloxane bond-containing methacrylate compound to be used as a main component. But, this results in falling into the dilemma that the oxygen permeability of the obtained copolymer is decreased.
It is an object of the present invention to provide a hard contact lens having an improved oxygen permeability.
A further object of the invention is to provide an oxygen permeable hard contact lens having an improved fragility as well as excellent hardness and rigidity.
These and other objects of the present invention will become apparent from the description hereinafter.